There's a guy named Alan Weiss[1], who is widely regarded as an expert (maybe the expert) on consulting. He has written several very popular books, including Million Dollar Consulting[2] and The Consulting Bible[3]. You may find his work useful. However, note that he would probably not classify what you're talking about (if I understand correctly) as "consulting" at all.
His take is that consultant is someone who shares their knowledge of process and works strategically with the client's decision makers... not someone who is knee deep in doing the work of implementing a project. If you're talking about writing PHP code, you may be more setting yourself up as a one man staffing agency, not as a consultant. I'd suggest reading Weiss, think hard about what you really want to do, and go from there.
His take is that consultant is someone who shares their knowledge of process and works strategically with the client's decision makers... not someone who is knee deep in doing the work of implementing a project. If you're talking about writing PHP code, you may be more setting yourself up as a one man staffing agency, not as a consultant. I'd suggest reading Weiss, think hard about what you really want to do, and go from there.
[1]: http://www.summitconsulting.com/about-alan/
[2]: http://www.amazon.com/Million-Dollar-Consulting-Alan-Weiss/d...
[3]: http://www.amazon.com/Consulting-Bible-Everything-Seven-Figu...